Reflection shadow mask alignment using coded apertures

ABSTRACT

In a shadow mask-substrate alignment method, a light source, a beam splitter, a first substrate including a first grate, a second substrate including a second grate, and a light receiver are positioned relative to each other to define a light path that includes light output by the light source being reflected a first time by the beam splitter. The light reflected the first time passes through the first or second grate and is at least partially reflected a second time by the second or first grate back through the first or the second grate, respectively. The light reflected the second time passes at least partially through the beam splitter for receipt by the light receiver. The orientation of the first substrate, the second substrate or both is adjusted to position the first grate, the second grate, or both until a predetermined amount is received by the light receiver.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.13/695,488, filed Oct. 31, 2012, which is the United States nationalphase of International Application No. PCT/US2011/037501, filed May 23,2011, which claims the benefit of U.S. Provisional Application No.61/351,470, filed Jun. 4, 2010. The disclosure of each of thesedocuments is hereby incorporated in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to accurately aligning a shadow mask and asubstrate in connection with the deposition of a material on thesubstrate in a vapor deposition system.

2. Description of Related Art

Accurate aligning of a shadow mask to a substrate in a vapor depositionsystem is critical to the accurate deposition of one or more materialson the substrate. Unfortunately, most vapor deposition systems includeenclosed vacuum deposition vessels where one or more vapor depositionevents occur and it is difficult to manually align a shadow mask to asubstrate with a high degree of accuracy. Moreover, current automatedand semi-automated systems for aligning a shadow mask to a substrate donot have the necessary alignment accuracy to provide a desired degree ofaccuracy when vapor depositing materials on the substrate, especiallywhen the substrate is subject to multiple vapor deposition events usingmultiple different shadow masks

It would, therefore, be desirable to provide a method and system ofshadow mask to substrate alignment that enables one or more materials tobe vapor deposited on the substrate via one or more shadow masks in ahighly accurate and repeatable manner

SUMMARY OF THE INVENTION

Disclosed herein is a shadow mask-substrate alignment method comprising:(a) positioning a collimated light source, a beam splitter, a substrateincluding a first grate, a shadow mask including a second grate, and alight receiver relative to each other to define a light path thatincludes collimated light output by the collimated light source being atleast partially reflected by the beam splitter, the at least partiallyreflected collimated light passing through one of the first or secondgrate and being at least partially reflected by the other of the firstor second grate back through the one of the first or second grate, andthe at least partially reflected light reflected back through the one ofthe first or second grate passing at least partially through the beamsplitter for receipt by the light receiver; and (b) causing theorientation of the substrate, the shadow mask or both to be adjusted toposition the first grate, the second grate, or both the first and secondgrates until a predetermined amount is received by the light receiver.

Each grate can include a plurality of spaced bars. A gap can separateeach pair of spaced bars. Each bar and each gap can have the same width.

Each grate can include a plurality of spaced bars and a gap separatingeach pair of spaced bars. Step (b) can include causing the orientationof the substrate, the shadow mask or both to be adjusted to positionelongated axes of the bars of the first grate parallel to elongated axesof the bars of the second grate and to position the bars of the firstgrate and the second grate to partially overlap the gaps of the secondgrate and the first grate, respectively.

The bars of the first and second grates can partially overlap the gapsof the second and first grates, respectively, by 50%.

The collimated light source can comprise an LED and a collimating lensoperative for collimating light output by the LED.

The light receiver can comprise a PIN diode and a focusing lensoperative for focusing light received from the beam splitter onto thePIN diode.

A longitudinal axis of each bar can extend radially ±15 degrees from acentral axis of the corresponding substrate or shadow mask.

Also disclosed herein is a shadow mask-substrate alignment methodcomprising: (a) providing a first substrate having a plurality of firstgrates in a pattern, wherein each first grate includes a plurality ofspaced bars and a gap between each pair of spaced bars; (b) providing asecond substrate having plural sets of spaced reflective surfaces in thesame pattern as the plurality of first grates, wherein each set ofspaced reflective surfaces includes a pair of spaced reflectivesurfaces; (c) defining a plurality of light paths, wherein each lightpath includes a light source and a light receiver at opposite ends ofthe light path, and a beam splitter in the light path between the lightsource and the light receiver; (d) positioning in each light path onefirst grate in coarse alignment with one set of spaced reflectivesurfaces; and (e) fine positioning the first substrate, the secondsubstrate, or both while light on each light path is received by thelight receiver of said light path after reflection and passage of saidlight through the beam splitter, reflection by at least one of thespaced reflective surfaces in said light path, and passage twice throughat least one gap in the first grate in said light path.

Each set of spaced reflective surfaces can be comprised of a secondgrate that includes a plurality of spaced bars and a gap between eachpair of spaced bars. Each bar of the second grate can define one of thereflective surfaces. Each gap of said second grate can define astructure of the second substrate that is less reflective than eachreflective surface.

Each light receiver can output a signal having a level related to anamount of light received by said light receiver. Step (e) can includefine positioning the first substrate, the second substrate, or bothuntil a combination of the levels of the signals output by the lightreceivers equals a predetermined value or falls within a predeterminedrange of values. The predetermined value can be zero.

The first substrate and the second substrate can each have a rectangularor square shape. The first substrate can have one first grate adjacenteach corner. The second substrate can have one set of spaced reflectivesurfaces adjacent each corner.

Also disclosed herein is a shadow mask-substrate alignment methodcomprising: (a) providing a substrate having a plurality of first gratesin a pattern; (b) providing a shadow mask having a plurality of secondgrates in the same pattern as the plurality of first grates, whereineach grate includes a plurality of spaced bars and a gap between eachpair of spaced bars; (c) defining a plurality of light paths, whereineach light path includes a light source, a light receiver and a beamsplitter; (d) positioning in each light path one first grate in coarsealignment with one second grate; and (e) fine positioning the substrate,the shadow mask, or both until a predetermined amount of light on eachlight path is received by the light receiver of said light path aftersaid light on said light path, output by the light source of said lightpath, is reflected by the beam splitter of said light path, passes afirst time through at least one gap in one of the first grate or secondgrate in said light path, is reflected by at least one bar of the otherof the first grate or second grate in said light path, passes a secondtime back through the at least one gap in the one first grate or secondgrate in said light path, and then passes through the beam splitter ofsaid light path for receipt by the light receiver of said light path.

Each bar and each gap can have the same width.

Step (e) can include fine positioning the substrate, the shadow mask, orboth until the bars of the first and second grates partially overlap thegaps of the second and first grates, respectively.

The bars of the first and second grates can partially overlap the gapsof the second and first grates, respectively, by 50%.

Each light receiver can output a signal having a level related to amountof light received by said light receiver. Step (e) can include finepositioning the substrate, the shadow mask, or both until a combinationof the levels of the signals output by the plurality of light receiversequals a predetermined value or falls within a predetermined range ofvalues. The predetermined value can be zero.

The substrate and the shadow mask can each have a rectangular or squareshape with one grate adjacent each corner of the rectangle or square. Alongitudinal axis of each bar can extend radially ±15 degrees from acentral axis of the corresponding substrate or shadow mask.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagrammatic illustration of a shadow mask depositionsystem for forming pixel structures of a high resolution OLED activematrix backplane;

FIG. 1B is an enlarged view of a single deposition vacuum vessel of theshadow mask deposition system of FIG. 1A;

FIG. 2 is a diagrammatic view of a first embodiment shadow maskalignment system;

FIG. 3A and FIG. 3B are plan views of an exemplary substrate and shadowmask, respectively, each of which includes a number of alignment gratesto facilitate orientation and positioning of the shadow mask to thesubstrate, or vice versa;

FIG. 4 is a view taken along lines IV-IV in FIG. 2;

FIG. 5 is a view taken along lines V-V in FIG. 2;

FIG. 6 is a diagrammatic view of a second embodiment shadow maskalignment system;

FIG. 7 is a view taken along lines VI-VI in FIG. 6; and

FIG. 8 is a view taken along lines VII-VII in FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1A and 1B, a shadow mask deposition system 2 forforming an electronic device, such as, without limitation, a highresolution active matrix organic light emitting diode (OLED) display,includes a plurality of serially arranged deposition vacuum vessels 4(e.g., deposition vacuum vessels 4 a-4 x). The number and arrangement ofdeposition vacuum vessels 4 is dependent on the number of depositionevents required for any given product to be formed therewith.

In one exemplary non-limiting use of shadow mask deposition system 2, acontinuous flexible substrate 6 translates through the serially arrangeddeposition vacuum vessels 4 by means of a reel-to-reel mechanism thatincludes a dispensing reel 8 and a take-up reel 10. Alternatively,substrate 6 can be a standalone (versus continuous) substrate that istranslated through serially arranged deposition vacuum vessels 4 by anysuitable means known in the art. Hereinafter, for the purpose ofdescribing the present invention, it will be assumed that substrate 6 isa standalone substrate.

Each deposition vacuum vessel includes a deposition source 12, asubstrate support 14, a shadow mask alignment system 15, and a shadowmask 16. For example, deposition vacuum vessel 4 a includes depositionsource 12 a, substrate support 14 a, mask alignment system 15 a, andshadow mask 16 a; deposition vacuum vessel 4 b includes depositionsource 12 b, substrate support 14 b, mask alignment system 15 b, andshadow mask 16 b; and so forth for any number of deposition vacuumvessels 4.

Each deposition source 12 is charged with a desired material to bedeposited onto substrate 6 through one or more openings in thecorresponding shadow mask 16 which is held in intimate contact with theportion of substrate 6 in the corresponding deposition vacuum vessel 4during a deposition event. Shadow mask 16 can be a conventional singlelayer shadow mask or a compound (multi-layer) shadow mask of the typedisclosed in U.S. Pat. No. 7,638,417 to Brody, which is incorporatedherein by reference.

Each shadow mask 16 of shadow mask deposition system 2 includes one ormore openings The opening(s) in each shadow mask 16 correspond(s) to adesired pattern of material to be deposited on substrate 6 from acorresponding deposition source 12 in a corresponding deposition vacuumvessel 4 as substrate 6 is translated through shadow mask depositionsystem 2.

Each shadow mask 16 can be formed of, for example, nickel, chromium,steel, copper, Kovar® or Invar®, and has a thickness desirably between20 and 200 microns, and more desirably between 20 and 50 microns. Kovar®and Invar® can be obtained from, for example, ESPICorp Inc. of Ashland,Oreg. In the United States, Kovar® is a registered trademark,Registration No. 337,962, currently owned by CRS Holdings, Inc. ofWilmington, Del., and Invar® is a registered trademark, Registration No.63,970, currently owned by Imphy S.A. Corporation of France.

Those skilled in the art will appreciate that shadow mask depositionsystem 2 may include additional stages (not shown), such as an annealstage, a test stage, one or more cleaning stages, a cut and mount stage,and the like, as are well-known. In addition, the number, purpose, andarrangement of deposition vacuum vessels 4 can be modified by one ofordinary skill in the art as needed for depositing one or more materialsin a desired order required for a particular application. An exemplaryshadow mask deposition system and method of use thereof is disclosed inU.S. Pat. No. 6,943,066 to Brody et al., which is incorporated herein byreference.

Deposition vacuum vessels 4 can be utilized for depositing materials onsubstrate 6 to form one or more electronic elements of an electronicdevice on substrate 6. Each electronic element may be, for example, athin film transistor (TFT), a memory element, a capacitor, etc. Acombination of one or more electronic elements can be deposited to forma higher level electronic element, such as, without limitation, asub-pixel or a pixel of the electronic device. As disclosed in U.S. Pat.No. 6,943,066 incorporated herein by reference, a multi-layer circuitcan be formed solely by successive depositions of materials on substrate6 via successive deposition events in deposition vacuum vessels 4.

Each deposition vacuum vessel 4 is connected to a source of vacuum (notshown) which is operative for establishing a suitable vacuum therein inorder to enable a charge of the material disposed in the correspondingdeposition source 12 to be deposited on substrate 6 in a manner known inthe art, e.g., sputtering or vapor phase deposition, through the one ormore openings in the corresponding shadow mask 16.

Regardless of the form of substrate 6, e.g., a continuous sheet or astandalone substrate, each deposition vacuum vessel 4 can includesupports or guides that avoid the sagging of substrate 6 as ittranslates therethrough.

In operation of shadow mask deposition system 2, the material disposedin each deposition source 12 is deposited on substrate 6 in thecorresponding deposition vacuum vessel 4 through one or more openings inthe corresponding shadow mask 16 in the presence of a suitable vacuum assaid substrate 6 is advanced through the deposition vacuum vessel 4,whereupon plural, progressive patterns is formed on substrate 6. Morespecifically, substrate 6 is positioned for a predetermined timeinterval in each deposition vacuum vessel 4. During this predeterminedtime interval, material is deposited from the corresponding depositionsource 12 onto substrate 6. After this predetermined time interval,substrate 6 is advanced to the next vacuum vessel in series foradditional processing, as applicable. This advancement continues untilsubstrate 6 has passed through all deposition vacuum vessels 4 whereuponsubstrate 6 exits the final deposition vacuum vessel 4 in the series.

With reference to FIG. 2 and with continuing reference to FIGS. 1A and1B, mask alignment system 15 includes one or more motion stages 20 forcontrolling the orientation and position of substrate 6, shadow mask 16,or both, to align substrate 6 and shadow mask 16 in a manner describedhereinafter. One desirable, non-limiting, embodiment of mask alignmentsystem 15 includes substrate 6 coupled to a Y-θ stage 20A and shadowmask 16 coupled to an X-Z stage 20B. The use of one or more stages 20 toeffect translation, orientation, and positioning of substrate 6, shadowmask 16, or both in the X direction, the Y direction, the Z direction,and/or the θ direction (in the present example the θ direction isrotational translation of substrate 6 in the X-Y plane) is well known inthe art and will not be described further herein for the purpose ofsimplicity.

Y-θ stage 20A and X-Z stage 20B are operated under the control of acontroller 22 which can be implemented by any suitable and/or desirablecombination of hardware and/or software to effect control of motionstages 20A and 20B in the manner described hereinafter.

Mask alignment system 15 further includes one or more light sources 24and one or more light receivers 26. Each light source 24 is positionedin alignment with one light receiver 26 to define a light source24-light receiver 26 pair. Each light source 24-light receiver 26 pairdefines a light path 36 therebetween.

In use of mask alignment system, substrate 6 and shadow mask 16 arepositioned in the light path 36 of each light source 24-light receiver26 pair. In one desirable embodiment, mask alignment system 15 includesfour light sources 24 and four light receivers 26, for a total of fourlight source 24-light receiver 26 pairs that define four light paths 36.However, this is not to be construed as limiting the invention.

With reference to FIGS. 3A-3B and with continuing reference to FIGS.1A-1B and 2, substrate 6 includes one or more grates 28 and shadow mask16 includes one or more grates 30. In one non-limiting embodiment,substrate 6 includes four grates 28A-28D and shadow mask 16 includesfour grates 30A-30D. In the embodiment shown in FIG. 3A, substrate 6 hasa rectangular or square shape and each grate 28A-28D is positionedadjacent one of the four corners of substrate 6. Similarly, shadow mask16 has a rectangular or square shape and each grate 30A-30D ispositioned adjacent one of the four corners of shadow mask 16. Thecentral portion of substrate 6 denoted by reference number 32 is wheredeposition events are to occur on substrate 6. The central portion ofshadow mask 16 denoted by the reference number 34 is where shadow mask16 includes a pattern of one or more openings where material from adeposition source 12 passes for deposit on area 32 in the same patternas the one or more openings of area 34 of shadow mask 16.

In the embodiment of substrate 6 shown in FIG. 3A, grate 28B is a mirrorimage of grate 28A about the Y-axis shown in FIG. 3A; and grates 28C and28D are mirror images of grates 28B and 28A, respectively, about theX-axis shown in FIG. 3A. However, this is not to be construed aslimiting the invention.

Similarly, in the embodiment of shadow mask 16 shown in FIG. 3B, grate30B is a mirror image of grate 30A about the Y-axis shown in FIG. 3B;and grates 30C and 30D are mirror images of grates 30B and 30A,respectively, about the X-axis shown in FIG. 3B. However, this is not tobe construed as limiting the invention.

The use of mask alignment system 15 to align substrate 6 having one ormore grates 28 and shadow mask 16 having one or more grates 30 will nowbe described.

Initially, substrate 6 is moved into spaced, coarse (or general)alignment with shadow mask 16 in the light paths 36 between lightsource(s) 24 and light receiver(s) 26 as shown in FIG. 2. When substrate6 and shadow mask 16 are in coarse alignment in light paths 36 as shownin FIG. 2, each grate 28 of substrate 6 and each grate 30 of shadow mask16 are positioned in one light path 36 of one light source 24—lightreceiver 26 pair. For example, where mask alignment system 15 includesfour light source—light receiver pairs 24A-26A, 24B-26B, 24C-26C, and24D-26D defining four light paths 36A-36D, respectively, and substrate 6includes grates 28A-28D, and shadow mask 16 includes grates 30A-30D:grates 28A and 30A are positioned in light path 36A that runs from lightsource 24A to light receiver 26A; grates 28B and 30B are positioned inlight path 36B that runs from light source 24B to light receiver 26B;grates 28C and 30C are positioned in light path 36C that runs from lightsource 24C to light receiver 26C; and grates 28D and 30D are positionedin light path 36D which runs from light source 24D to light receiver26D.

FIG. 4 is a top down view of substrate 6 in coarse alignment with shadowmask 16 between light sources 24A-24D and light receivers 26A-26D (shownin phantom) and the position of light paths 36A-36D for each lightsource—light receiver pair, respectively. In FIG. 4 it is to beunderstood that grates 28A and 30A are positioned in light path 36A;grates 28B and 30B are positioned in light path 36B; grates 28C and 30Care positioned in light path 36C; and grates 28D and 30D are positionedin light path 36D.

With reference to FIG. 5, the fine alignment of one grate 28 ofsubstrate 6 and one grate 30 of shadow mask 16 (i.e., one grate pair28-30) lying along one light path 36 will now be described. It is to beunderstood, however, that the fine alignment of the grate pair 28-30lying along the light path 36 shown in FIG. 5 is also applicable to thealignment of each grate pair 28-30 positioned in each light path 36.

At a suitable time, each light source 24 is activated to output lightalong its light path 36. In one, non-limiting, embodiment each lightsource includes an LED 38 which outputs light to a collimator optic/lens40 which collimates the light output by LED 38 and outputs saidcollimated light along light path 36.

Each grate 28 of substrate 6 includes a plurality of spaced bars 42,desirably spaced parallel bars. Each pair of spaced bars 42 is separatedby a gap 44. Desirably, the width of each gap 44 is the same. Similarly,each grate 30 includes a plurality of spaced bars 46, desirably spacedparallel bars. Each pair of spaced bars 46 is separated by a gap 48.Desirably, the width of each gap 48 is the same. Desirably, the width ofeach gap 44 and each gap 48 are also the same. However, the widths ofthe gaps being the same in grate 28, grate 30, or both grates 28 and 30is not to be construed as limiting the invention.

With continuing reference to FIG. 5 and with reference back to FIGS. 3Aand 3B, it is not necessary that each bar or each gap of substrate 6 andshadow mask 16 be oriented or positioned at the same angle relative toits respective X axis. For example, a longitudinal axis of each bar 42and each gap 44 of substrate 6 is desirably, nominally oriented orpositioned at an angle θ1 of 45 degrees with respect to the X axis shownin FIG. 3A. However, the orientation angle θ1 of the longitudinal axisof each bar 42 and each gap 44 can vary ±15 degrees with respect to thenominal orientation angle θ1 of 45 degrees with respect to the X axis.Moreover, each bar 42 and each gap 44 can be oriented or positioned at adifferent angle θ1. Desirably, however, the bars 42 and gaps 44 of eachgrate of substrate 6 are parallel.

Similarly, a longitudinal axis of each bar 46 and each gap 48 of shadowmask 16 is desirably, nominally oriented or positioned at an angle θ2 of45 degrees with respect to the X axis shown in FIG. 3B. However, theorientation angle θ2 of the longitudinal axis of each bar 46 and eachgap 48 can vary ±15 degrees with respect to the nominal orientationangle θ2 of 45 degrees with respect to the X axis. Moreover, each bar 46and each gap 48 can be oriented or positioned at a different angle θ2.Desirably, however, the bars 46 and gaps 48 of each grate of shadow mask16 are parallel.

More generally, the longitudinal axis of each bar 42, 46, and thelongitudinal axis of each gap 44 and 48 desirably extends radially ±15degrees from a center of substrate 6 and shadow mask 16, respectively.Desirably, for each grate, the bars and gaps of said grate are parallel.However, it is also envisioned that the bars and gaps of said grate canextend radially in a spoke-like pattern from the center of substrate 6or shadow mask 16, as may be the case. Thus, where angles θ1-θ2 are, forexample, without limitation, oriented or positioned at an angle of 30degrees with respect to the corresponding X axis, the longitudinal axisof each bar 42, 46, and the longitudinal axis of each gap 44 and 48 canvary from 30 degrees by ±15 degrees.

It should be appreciated that it is possible for the angulardisplacement between bars 42 and 46 and gaps 48 and 44 of any grate pair28-30 to vary by as much as 30 degrees, e.g., when a bar 42 ispositioned at an angle of 60 degrees with respect to the X axis of itssubstrate 6, a gap 48 is positioned at an angle of 30 degrees withrespect to the X axis of its shadow mask 16, and the X axes of thesubstrate 6 and the shadow mask 16 are parallel; the difference between60 degrees and 30 degrees being 30 degrees.

Collimated light output by light source 24 passes through the gaps 44and 48 of coarsely aligned grates 28 and 30, respectively, and isreceived by light receiver 26. Light receiver 26 includes a focusingoptics/lens 50 which focuses the collimated light after passage throughthe coarsely aligned gaps 44 and 48 of grates 28 and 30 for receipt by alight detection means in the form of a PIN diode 52. The output of eachPIN diode 52 of each light receiver 26 of mask alignment system 15 isprovided to an analog-to-digital (A/D) convertor 54 of controller 22which converts the analog output of each PIN diode 52 into acorresponding digital signal for processing by a processing means ofcontroller 22. The output of each PIN diode 52 corresponds to the amountof light received by the PIN diode 52—the greater the amount of lightreceived by the PIN diode 52 the greater its output voltage, the lesserthe amount of light received by the PIN diode 52 the lesser its outputvoltage.

At a suitable time, controller 22 commences fine positioning ofsubstrate 6, shadow mask 16, or both via Y-θ stage 20A and/or X-Z stage20B to align substrate 6 and shadow mask 16 relative to each other suchthat, for each grate pair 28-30 positioned in a light path 36, at leastsome of the bars 42 of grate 28 overlap (in a direction transverse,desirably perpendicular to light path 36) some of the gaps 48 of grate30 to a desired extent, and at least some of the bars 46 of grate 30overlap (in a direction transverse, desirably perpendicular to lightpath 36) some of the gaps 44 of grate 28 to a desired extent. Desirably,each gap 48 of shadow mask 16 is partially overlapped by a bar 42 ofsubstrate 6 and each gap 44 of substrate 6 is partially overlapped by abar 46 of shadow mask 16 as shown in FIG. 5. More desirably, bars 42 and46 partially overlap the width of gaps 48 and 44, respectively, by 50%.In other words, 50% of the width of gaps 48 and 44 is overlapped by bars42 and 46.

For each grate pair 28-30 positioned in one of the light paths 36,controller 22 detects when bars 42 and 46 overlap gaps 48 and 44,respectively, to a desired extent, by comparing the digitized output ofthe PIN diode 52 on said light path 36 (which digitized output isobtained via A/D 54 and which digitized output corresponds to thecollimated light passing through gaps 48 and 44) to a predeterminedvalue or a predetermined range of values. Upon detecting that thedigitized output of the PIN diode 52 is not at the predetermined valueor within the predetermined range of values, controller 22 causes one ormore motion stages 20A and 20B to adjust the X, Y, and/or θ position ofsubstrate 6, shadow mask 16, or both, as necessary until a desiredamount of overlap between the bars 42 and 46 overlap gaps 48 and 44,respectively, of the grate pair 28-30 is detected by controller 22 viathe digitized output of PIN diode 52. Since the amount of overlapbetween bars 42 and 46 and gaps 48 and 44, respectively, of the gratepair 28-30 affects the amount of collimated light that reaches PIN diode52, by comparing the digitized output of PIN diode 52 to thepredetermined value or the predetermined range of values, controller 22can determine when an appropriate amount of overlap of the bars and gapsof the grate pair 28-30 in light path 36 has been achieved. In a similarmanner, controller 22 can determine when an appropriate amount ofoverlap of the bars and gaps of each other grate pair 28-30 in eachother light path 36 has been achieved.

In one non-limiting embodiment, controller 22 desirably combines theoutput of all of the PIN diodes 52 of light receivers 26A-26D todetermine when proper X, Y, and θ alignment between substrate 6 andshadow mask 16 has been achieved. More specifically, suppose controller22 adjusts the orientation/position of substrate 6, shadow mask 16, orboth. After some period of time, controller 22 stops adjusting theorientation/position of substrate 6, shadow mask 16, or both, and causesA/D 54 to sample and digitize the outputs of PIN diodes 52A-52D (shownin FIG. 4) of light receivers 26A-26D. Controller 22 associates in amemory of controller 22 the digitized outputs of PIN diodes 52A-52D withvariables f1-f4 and combines these variables for the X, Y, androtational or angular (θ) displacements of substrate 6, shadow mask 16,or both as follows:X displacement=f1−f2−f3+f4   (Equation 1)Y displacement=f1+f2−f3−f4   (Equation 2); andθ displacement=f1−f2+f3−f4   (Equation 3).

Upon controller 22 determining that the X, Y, and θ displacementsdetermined by Equations 1-3 above each equals 0, controller 22recognizes this condition as corresponding to substrate 6 and shadowmask 16 having a desired alignment. On the other hand, if any one of theX displacement, Y displacement, or θ displacement is not equal to 0,controller 22 recognizes this condition as corresponding to substrate 6and shadow mask 16 NOT having a desired alignment, whereupon controller22 causes the one or more motion stages 20A-20B to adjust the X, Y,and/or θ position(s) of substrate 6, shadow mask 16, or both, asnecessary to cause the X displacement, Y displacement, or θ displacementdetermined by Equations 1-3 above to each equal 0.

Desirably, controller 22 repeats the foregoing steps of: adjusting theorientation/position of substrate 6, shadow mask 16, or both; stoppingthe adjusting of the orientation/position of substrate 6, shadow mask16, or both; sampling and digitizing the outputs of PIN diodes 52A-52D;and determining whether the X, Y, and θ displacements determined byEquations 1-3 above each equals 0 until the X, Y, and θ displacementsdetermined by Equations 1-3 above in fact each equals 0, a predeterminednumber of repetitions of said steps has occurred, or a predeterminedamount of time has elapsed.

Upon determining that the X, Y, and θ displacements each equals 0,controller 22 causes the motion stage 20 that moves in the Z directionto move substrate 6 and shadow mask 16 into intimate contact from theposition in spaced relationship shown in FIG. 5, which spaced relationis used for the purpose of aligning substrate 6 and shadow mask 16.

The determination of the X, Y, and θ displacements using Equations 1-3in the above-described manner to each equal 0, however, is not to beconstrued as limiting the invention since it is envisioned that eachdisplacement can be within a range of suitable values unique to saiddisplacement or common to all of said displacements. For example,without limitation, controller 22 can be programmed such that an Xdisplacement that falls within a range of ±1 is acceptable, that a range±1.5 for the Y displacement value is acceptable, and that a range of±0.5 for the θ displacement is acceptable. Alternatively, controller 22can be programmed to use the same range of values for each displacement.For example, controller 22 may be programmed such that it is acceptableto have each of the X, Y, and θ displacements fall within a range of ±1.

As can be seen, by utilizing the output of the PIN diodes 52A-52D oflight receivers 26A-26D, controller 22 can position substrate 6 andshadow mask 16 in a desired state of alignment with a high degree ofaccuracy. To this end, controller 22 can incrementally orient/positionsubstrate 6, shadow mask 16, or both, until the grates 28 of substrate 6and the grates 30 of shadow mask 16 are aligned to a desired extent. Inthe event controller 22 determines that further alignment of substrate 6and shadow mask 16 is needed, controller 22 can make an informeddecision from the values of the X, Y, and θ displacements determinedusing Equations 1-3 above which way to move or rotate substrate 6,shadow mask 16, or both, in the X, Y, and θ directions as necessary toimprove the alignment of substrate 6 and shadow mask 16. Thus,controller 22 can orient/position substrate 6, shadow mask 16, or both,in a first position and then acquire the output of the PIN diodes52A-52D of light receivers 26A-26D to determine if substrate 6 andshadow mask 16 are properly aligned. If so, controller 22 causessubstrate 6 and shadow mask 16 to move in the Z direction into intimatecontact in preparation for a deposition event occurring in depositionvacuum vessel 4. However, if substrate 6 and shadow mask 16 aredetermined to not be in proper alignment, controller 22 canincrementally orient/position substrate 6, shadow mask 16, or both, toanother position, where controller 22 samples the outputs of the PINdiodes 52A-52D of light receivers 26A-26D. The process of sampling theoutputs of PIN diodes 52A-52D of light receivers 26A-26D andincrementally orienting/positioning substrate 6, shadow mask 16, orboth, continues until controller 22 determines that substrate 6 andshadow mask 16 are aligned to a desired extent determined by theprogramming of controller 22.

As can be further seen, controller 22 causes the orientation of thesubstrate 6, the shadow mask 16, or both, to be adjusted to position thegrates 28 of substrate 6 and the grates 30 of shadow mask 16, or both,until a predetermined amount of collimated light on each light path 36passes through the grates that lie on said light path 36 for receipt bythe corresponding light receiver 26. Stated differently, controller 22fine positions substrate 6, shadow mask 16, or both, until apredetermined amount of light on each light path 36 passes through thegrates in said path and is received by the light receiver on said lightpath.

With reference to FIG. 6, another embodiment shadow mask alignmentsystem 15′ is similar in all respects to mask alignment system 15 shownin FIG. 2 with the following exceptions: each light source 24-lightreceiver 26 pair is positioned to the same side of substrate 6 andshadow mask 16; and each light source 24-light receiver 26 pair togetherwith a beam splitter 60 defines a light path 36′. More specifically,each grate 28 of substrate 6 and each grate 30 of shadow mask 16 arepositioned in one light path 36′ of one light source 24-light receiver26 pair shown in FIG. 6. Each light path 36′ runs from one light source24 for reflection by one of the beam splitters 60 which reflects thelight from the light source 24 toward a grate 28 via a grate 30 ofshadow mask 30 of substrate 6 in alignment with said grate 28 ofsubstrate 6. Some of the light impinging on grate 28 of substrate 6 isreflected by said gate 28 back through grate 30 of shadow mask 16 inalignment with said grate 28 toward beam splitter 60. Beam splitter 60passes some of the light reflected from grate 28 of substrate 6 afterpassage through grate 30 of shadow mask 16 in alignment with said grate28 to the light receiver 26 at the terminal end of said light path 36′.

The foregoing description assumed that light source 24-light receiverpair 26, and beam splitter 60 are positioned on a side of shadow mask 16opposite substrate 6. However, it is envisioned that light source24-light receiver pair 26 and beam splitter 60 can be positioned on theother side of substrate 6, whereupon the light from light source 24reflected by beam splitter 60 first passes through a grate 28 ofsubstrate 6 before partial reflection by a grate 30 of shadow mask 16and passage of said partially reflected light back through said grate 28toward beam splitter 60 which passes some of the reflected light passingthrough grate 30 to light receiver 26. Accordingly, the foregoingdescription and illustration in FIG. 6 of light source 24-light receiver26 pair and beam splitter 60 being positioned on a side of shadow mask16 opposite substrate 6 is not to be construed as limiting theinvention.

The use of mask alignment system 15′ to align substrate 6 having one ormore grates 28 and shadow mask 16 having one or more grates 30 will nowbe described.

Initially, substrate 6 is moved into spaced, coarse (or general)alignment with shadow mask 16. When substrate 6 and shadow mask 16 arein coarse alignment, each grate 28 of substrate 6 and each grate 30 ofshadow mask 16 are positioned in the light path 36′ of one light source24-light receiver 26 pair. For example, where mask alignment system 15′includes four light source-light receiver pairs 24A-26A, 24B-26B,24C-26C, and 24D-26D (shown schematically in FIG. 7) defining four lightpaths 36A′, 36B′, 36C′, and 36D′ (also shown in FIG. 7), respectively,and substrate 6 includes grates 28A, 28B, 28C, and 28D, and shadow mask16 includes grates 30A, 30B, 30C, and 30D: grates 28A and 30A arepositioned in light path 36A′ that runs from light source 24A to lightreceiver 26A via beam splitter 60A; grates 28B and 30B are positioned inlight path 36B′ that runs from light source 24B to light receiver 26Bvia beam splitter 60B; grates 26C and 30C are positioned in light path36C′ that runs from light source 24C to light receiver 26C via beamsplitter 60C; and grates 28D and 30D are positioned in light path 36D′which runs from light source 24D to light receiver 26D via beam splitter60D.

FIG. 7 is a top down schematic view of substrate 6 in coarse alignmentwith shadow mask 16 showing light sources 24A-24D, light receivers26A-26D (shown in phantom), beam splitters 60A-60D (shown in phantom),and light paths 36A′36D′ for each light source-light receiver pair,respectively. In FIG. 7, it is to be understood that grates 28A and 30Aare positioned in light path 36A′; grates 28B and 30B are positioned inlight path 36B′; grates 26C and 30C are positioned in light path 36C′;and grates 28D and 30D are positioned in light path 36D′.

With reference to FIG. 8, the fine alignment of one grate 28 ofsubstrate 6 and one grate 30 of shadow mask 16 (i.e., one grate pair28-30) lying along one light path 36′ will now be described. It is to beunderstood, however, that fine alignment of the grate pair 28-30 lyingalong light path 36′ shown in FIG. 8 is also applicable to the alignmentof each other grate pair 28-30 positioned in each other light path 36′.

Starting with shadow mask 16 and substrate 6 in spaced relation, at asuitable time, each light source 24 is activated to output light alongits light path 36′. In one, non-limiting, embodiment each light sourceincludes LED 38 which outputs light to collimator optics/lens 40 whichcollimates the light output by LED 38 and outputs said collimated lightto beam splitter 60 which reflects at least a portion of the collimatedlight output by collimator optics/lens 40 toward one grate pair 28-30.

As discussed above, each grate 30 of shadow mask 16 includes a pluralityof spaced bars 46, desirably spaced parallel bars. Each pair of spacedbars 46 is separated by a gap 48. Desirably, the width of each gap 48 isthe same. Similarly, each grate 28 of substrate 6 includes a pluralityof spaced bars 42, desirably spaced parallel bars. Each pair of spacedbars 42 is separated by a gap 44. Desirably the width of each gap 44 isthe same. Moreover, the widths of each gap 44 and each gap 48 are alsodesirably the same. However, the widths of the gaps 44, 48 being thesame in grate 28, grate 30, or both grates 28 and 30, is not to beconstrued as limiting the invention.

Collimated light from beam splitter 60 that passes through gaps 48 ofgrate 30 and gaps 44 of grate 28 does not contribute to the lightreflected by spaced bars 42 of grate 28. However, collimated lightreflected by spaced bars 42 of grate 28 passes back through gaps 48 ofgrate 30 and propagates toward beam splitter 60. At beam splitter 60, atleast part of the reflected light passes through beam splitter 60 forreceipt by light receiver 26.

Focusing optics/lens 50 of light receiver 26 focuses the part of thereflected light that passes through beam splitter 60 toward the lightdetection means in the form of PIN diode 52. The output of each PINdiode 52 of each light receiver 26 of mask alignment system 15′ isprovided to analog to digital (A/D) converter 54 of controller 22 (shownin FIG. 6) which converts the analog output of each PIN diode 52 into acorresponding digital signal for processing by a processing means ofcontroller 22. The output of each PIN diode 52 corresponds to the amountof light received by said PIN diode 52.

At a suitable time, controller 22 commences fine positioning ofsubstrate 6, shadow mask 16, or both, via Y-θ stage 20A and/or X-Z stage20B to align substrate 6 and shadow mask 16 relative to each other suchthat, for each light path 36′, some of the bars 42 of grate 28 overlap(in a direction transverse, desirably perpendicular to light path 36′)some of the gaps 48 of grate 30 to a desired extent, and, conversely, atleast some of the bars 46 of grate 30 overlap (in a directiontransverse, desirably perpendicular to light path 36′) some of the gaps44 of grate 28 to a desired extent. Desirably, each gap 48 of shadowmask 16 is partially overlapped by a bar 42 of substrate 6, and each gap44 of substrate 6 is partially overlapped by a bar 46 of shadow mask 16as shown in FIG. 8. More desirably, bars 42 and 46 partially overlap thewidths of gaps 48 and 44, respectively, by a desired percentage, e.g.,50%. In other words, 50% of the width of gaps 48 and 44 are overlappedby bars 42 and 46, respectively.

For each grate pair 28-30 associated with one of the light paths 36′,controller 22 detects when bars 42 overlap gaps 48 to a desired extentby comparing the digitized output of the PIN diode 52 on said light path36′, which digitized output is obtained via A/D 54 and which digitizedoutput corresponds to the light reflected from bars 42 passing throughgaps 48, to a predetermined value or a predetermined range of values.

Upon detecting that the digitized output of the PIN diode 52 is not atthe predetermined value or within the predetermined range of values,controller 22 causes the one or more motion stages 20A and/or 20B toadjust the X, Y, and/or θ position of substrate 6, shadow mask 16 orboth, as necessary until a desired amount of overlap between bars 42 andgaps 48 of the grate pair 28-30 is detected by controller 22 via thedigitized output of PIN diode 52. Since the amount of overlap betweenbars 42 and gaps 48 of the grate pair 28-30 affects the amount ofcollimated light that reaches PIN diode 52, by comparing the digitizedoutput of PIN diode 52 to the predetermined value or the predeterminedrange of values, controller 22 can determine when an appropriate amountof overlap of the bars 42 and gaps 48 of the grate pair 28-30 associatedwith the light path 36′ has been achieved. In a similar manner,controller 22 can determine when an appropriate amount of overlap of thebars 42 and gaps 48 of each other grate pair 28-30 associated with eachother light path 36′ has been achieved.

In one non-limiting embodiment, controller 22 desirably combines theoutput of all of the pin diodes 52 of light receivers 26A-26D todetermine when proper X, Y, and θ alignment between substrate 6 andshadow mask 16 has been achieved. More specifically, suppose controlleradjusts the orientation/position of substrate 6, shadow mask 16, orboth. After some period of time, controller stops adjusting theorientation/position of substrate 6, shadow mask 16, or both, and causesA/D 54 to sample and digitize the outputs of pin diodes 52A-52D (shownin FIG. 7) of light receivers 26A-26D. Controller 22 associates in amemory of controller 22 the digitized output of pin diodes 52A-52D withvariables f1-f4 and combines these variables for the X, Y, androtational or angular (θ) displacements of substrate 6, shadow mask 16,or both, as follows:X displacement=f1−f2−f3+f4   (Equation 1)Y displacement=f1+f2−f3−f4   (Equation 2); andθ displacement=f1−f2+f3−f4   (Equation 3).Equations 1-3 immediately above are copies of equations 1-3 from thediscussion of the first embodiment mask alignment system 15 discussedpreviously.

Upon controller 22 determining that the X, Y, and θ displacementsdetermined by Equations 1-3 above each equals 0, controller 22recognizes this condition as corresponding to substrate 6 and shadowmask 16 having a desired alignment. On the other hand, if any one of theX displacement, Y displacement, or θ displacement is not equal to 0,controller 22 recognizes this condition as corresponding to substrate 6and shadow mask 16 NOT having a desired alignment, whereupon controller22 causes the one or more motion stages 20A-20B to adjust the X, Y,and/or θ position(s) of substrate 6, shadow mask 16, or both, asnecessary to cause the X displacement, Y displacement, or θ displacementdetermined by Equations 1-3 above to each equal 0.

Desirably, controller 22 repeats the foregoing steps of: adjusting theorientation/position of substrate 6, shadow mask 16, or both; stoppingthe adjusting of the orientation/position of substrate 6, shadow mask16, or both; sampling and digitizing the outputs of PIN diodes 52A-52D;and determining whether the X, Y, and θ displacements determined byEquations 1-3 above each equals 0 until the X, Y, and θ displacementsdetermined by Equations 1-3 above in fact each equals 0; or until apredetermined number of repetitions of said steps has occurred; or untila predetermined amount of time has elapsed.

Upon determining that the X, Y, and θ displacements each equals 0,controller 22 causes the motion stage 20B that moves in the Z directionto move substrate 6 and shadow mask 16 into intimate contact from theposition in spaced relationship shown in FIG. 8, which spaced relationis used for the purpose of aligning substrate 6 and shadow mask 16.

The determination of the X, Y, and θ displacements using Equations 1-3in the above-described manner to each equal 0 is not to be construed aslimiting the invention since it is envisioned that each displacement canbe within a range of suitable values unique to said displacement orcommon to all of said displacements. For example, without limitation,controller 22 can be programmed such that an X displacement that fallswithin a range of ±1 is acceptable, that a range ±1.5 for the Ydisplacement value is acceptable, and that a range of ±0.5 for the θdisplacement is acceptable. Alternatively, controller 22 can beprogrammed to use the same range of values for each displacement. Forexample, controller 22 may be programmed such that it is acceptable tohave each of the X, Y, and θ displacements each fall within a range of±1.

As can be seen, by utilizing the output of the PIN diodes 52A-52D oflight receivers 26A-26D, controller 22 can position substrate 6 andshadow mask 16 in a desired state of alignment with a high degree ofaccuracy. To this end, controller 22 can incrementally orient/positionsubstrate 6, shadow mask 16, or both, until the grates 28 of substrate 6and the grates 30 of shadow mask 16 are aligned to a desired extent. Inthe event controller 22 determines that further alignment of substrate 6and shadow mask 16 is needed, controller 22 can make an informeddecision from the values of the X, Y, and θ displacements determinedusing Equations 1-3 above which way to move or rotate substrate 6,shadow mask 16, or both, in the X, Y, and θ directions as necessary toimprove the alignment of substrate 6 and shadow mask 16. Thus,controller 22 can orient/position substrate 6, shadow mask 16, or both,in a first position and then acquire the output of the PIN diodes52A-52D of light receivers 26A-26D to determine if substrate 6 andshadow mask 16 are properly aligned. If so, controller 22 causessubstrate 6 and shadow mask 16 to move in the Z direction into intimatecontact in preparation for a deposition event occurring in depositionvacuum vessel 4. However, if substrate 6 and shadow mask 16 aredetermined to not be in proper alignment, controller 22 canincrementally orient/position substrate 6, shadow mask 16, or both, toanother position, where controller 22 samples the outputs of the PINdiodes 52A-52D of light receivers 26A-26D. The process of sampling theoutputs of PIN diodes 52A-52D of light receivers 26A-26D andincrementally orienting/positioning substrate 6, shadow mask 16, orboth, can continue until controller 22 determines that substrate 6 andshadow mask 16 are aligned to a desired extent determined by theprogramming of controller 22.

As can be seen, controller 22 causes the orientation of substrate 6,shadow mask 16, or both, to be adjusted to position grates 28 ofsubstrate 6 and grates 30 of shadow mask 16, or both, until apredetermined amount of reflected light on each light path 36′ passesthrough the corresponding beam splitter 60 for receipt by thecorresponding light receiver 26. In other words, controller finepositions substrate 6, shadow mask 16, or both, until a predeterminedamount of light reflected by bars 42 of each grate 28 passes throughgaps 48 of each grate 30 and beam splitter 60 for receipt by thecorresponding light receiver 26.

As shown in FIG. 8, in mask alignment system 15′, light sources 24A-24Dare positioned on the same side of shadow mask 16 as light receivers26A-26D, respectively. Each light source 24-light receiver 26 pair iscombined optically by utilizing beam splitter 60 (shown as a 45 degreefacet) as a beam combiner.

Each light receiver 26 is positioned with its optical axis normal toshadow mask 16 and substrate 6 and is positioned to a side of shadowmask 16 opposite substrate 6. However, this is not to be construed aslimiting the invention.

In the arrangement shown in FIG. 8, light from beam splitter 60 passesthrough gaps 48 of shadow mask 16 and is at least partially reflected bybars 42 of substrate 6, with a remainder of the light from beam splitter60 passing through gaps 44 of substrate 6. The light reflected by bars42 of substrate 6 passes once again through gaps 48 of shadow mask 16and then at least partially through beam splitter 60 for receipt bylight receiver 26.

Light source 24 including collimator optics/lens 40 is positionedtransverse, desirably perpendicular, to light receiver 26 includingfocusing optics/lens 50. Beam splitter 60 is placed at the intersectionof the optical axes of light source 24 and light receiver 26 with theplane of beam splitter 60 oriented to bisect at right angle the axes oflight source 24 and light receiver 26. In this way, the collimated lightfrom light source 24 is reflected by 90 degrees whereupon the reflectedlight propagates through gaps 48 of shadow mask 16 a first time toimpinge upon and be at least partially reflected by the downward facingsurfaces of bars 42 of grate 28 of substrate 6. The light reflected bythe bars 42 of grate 28 passes through gaps 48 of shadow mask 16 asecond time and then partially through beam splitter 60 for receipt bythe PIN diode 52 of light receiver 26. The amount of light received bythe PIN diode 52 of light receiver 26 is thus proportional to the degreeof overlap between gaps 48 of grate 30 of shadow mask 16 and bars 42 ofgrate 28 of substrate 6. Thus, mask alignment 15′ performs alignmentsensing in a reflection mode. This is in contrast to mask alignmentsystem 15 discussed above performing alignment sensing in a transmissionmode.

One skilled in the art would readily recognize that some amount of lightis discarded because of the partial use of light due to the partialreflection and partial transmission of beam splitter 60. It is wellunderstood that the inefficiency of beam splitter 60 can be accountedfor by use of collimator optics/lens 40 and beam splitter 60 being apolarizing beam splitter.

In the above-described embodiment of mask alignment system 15′, eachlight source 24-light receiver 26 pair and each corresponding beamsplitter 60 is positioned to a side of shadow mask 16 opposite substrate6. However, this is not to be construed as limiting the invention sinceit is envisioned that one, or more, or all of said light source 24-lightreceiver 26 pairs and each corresponding beam splitter 60 can bepositioned to a side of substrate 6 opposite shadow mask 16, i.e., onthe other side of substrate 6. In this embodiment, light from each beamsplitter 60 would first pass through the gaps 44 of substrate 6 forreflection by the bars 46 of shadow mask 16, which reflected light wouldthen pass again through gaps 44 of substrate 6 for subsequent passagethrough beam splitter 60 for receipt by light receiver 26.

The invention has been described with reference to exemplaryembodiments. Obvious modifications and alterations will occur to othersupon reading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

The invention claimed is:
 1. A shadow mask-substrate alignment methodcomprising: (a) positioning a collimated light source, a beam splitter,a substrate including a first grate, a shadow mask including a secondgrate, and a light receiver relative to each other to define a lightpath that includes collimated light output by the collimated lightsource being at least partially reflected by the beam splitter, the atleast partially reflected collimated light passing through one of thefirst or second grate and being at least partially reflected by theother of the first or second grate back through the one of the first orsecond grate, and the at least partially reflected light reflected backthrough the one of the first or second grate passing at least partiallythrough the beam splitter for receipt by the light receiver; and (b)causing the orientation of the substrate, the shadow mask or both to beadjusted to position the first grate, the second grate, or both thefirst and second grates until a predetermined amount is received by thelight receiver, wherein: each grate includes a plurality of bars inspaced relation; each pair of spaced bars is separated by a gap; and atleast one of the first grate and the second grate includes at least onegap that extends through the respective substrate and shadow mask. 2.The method of claim 1, wherein each bar and each gap has the same width.3. The method of claim 1, wherein: each grate includes a plurality ofspaced bars and a gap separating each pair of spaced bars; and step (b)includes causing the orientation of the substrate, the shadow mask orboth to be adjusted to position elongated axes of the bars of the firstgrate parallel or substantially parallel to elongated axes of the barsof the second grate and to position the bars of the first and secondgrates to partially overlap the gaps of the second and first grates,respectively.
 4. The method of claim 3, wherein the bars of the firstand second grates partially overlap the gaps of the second and firstgrates, respectively, by 50%.
 5. The method of claim 1, wherein thecollimated light source comprises: an LED; and a collimating lensoperative for collimating light output by the LED.
 6. The method ofclaim 1, wherein the light receiver comprises: a PIN diode; and afocusing lens operative for focusing light received from the beamsplitter onto the PIN diode.
 7. The method of claim 1, wherein alongitudinal axis of each bar extends radially ±15 degrees from acentral axis of the corresponding substrate or shadow mask.
 8. A shadowmask-substrate alignment method comprising: (a) providing a firstsubstrate having a plurality of first grates in a pattern, wherein eachfirst grate includes a plurality of spaced bars and a gap through thefirst substrate between each pair of spaced bars; (b) providing a secondsubstrate having plural sets of spaced reflective surfaces in the samepattern as the plurality of first grates, wherein each set of spacedreflective surfaces includes a pair of spaced reflective surfaces; (c)defining a plurality of light paths, wherein each light path includes alight source and a light receiver at opposite ends of the light path,and a beam splitter in the light path between the light source and thelight receiver; (d) positioning in each light path one first grate incoarse alignment with one set of spaced reflective surfaces; and (e)fine positioning the first substrate, the second substrate, or bothwhile light on each light path is received by the light receiver of saidlight path after reflection and passage of said light through the beamsplitter, reflection by at least one of the spaced reflective surfacesin said light path, and passage twice through at least one gap in thefirst grate in said light path.
 9. The method of claim 8, wherein: eachset of spaced reflective surfaces is comprised of a second grate thatincludes a plurality of spaced bars and a gap between each pair ofspaced bars; each bar of said second grate defines one of the reflectivesurfaces; and each gap of said second grate defines a structure of thesecond substrate that is less reflective than each reflective surface.10. The method of claim 8, wherein: each light receiver outputs a signalhaving a level related to an amount of light received by said lightreceiver; and step (e) includes fine positioning the first substrate,the second substrate, or both until a combination of the levels of thesignals output by the light receivers equals a predetermined value orfalls within a predetermined range of values.
 11. The method of claim10, wherein the predetermined value is zero.
 12. The method of claim 8,wherein: the first substrate and the second substrate each have arectangular or square shape; the first substrate has one first grateadjacent each corner; and the second substrate has one set of spacedreflective surfaces adjacent each corner.
 13. A shadow mask-substratealignment method comprising: (a) providing a substrate having aplurality of first grates in a pattern; (b) providing a shadow maskhaving a plurality of second grates in the same pattern as the pluralityof first grates, wherein each grate includes a plurality of spaced barsand a gap between each pair of spaced bars, and at least one of thefirst grate and the second grate includes at least one gap that extendsthrough the respective substrate and shadow mask; (c) defining aplurality of light paths, wherein each light path includes a lightsource, a light receiver and a beam splitter; (d) positioning in eachlight path one first grate in coarse alignment with one second grate;and (e) fine positioning the substrate, the shadow mask, or both until apredetermined amount of light on each light path is received by thelight receiver of said light path after said light on said light path,output by the light source of said light path, is reflected by the beamsplitter of said light path, passes through at least one gap in one ofthe first or second grates in said light path, is reflected by at leastone bar of the other of the first or second grates in said light pathand passes back through the at least one gap in the one first or secondgrate in said light path and then passes through the beam splitter ofsaid light path for receipt by the light receiver of said light path.14. The method of claim 13, wherein each bar and each gap has the samewidth.
 15. The method of claim 13, wherein step (e) includes finepositioning the substrate, the shadow mask, or both until the bars ofthe first and second grates partially overlap the gaps of the second andfirst grates, respectively.
 16. The method of claim 15, wherein the barsof the first and second grates partially overlap the gaps of the secondand first grates, respectively, by 50%.
 17. The method of claim 13,wherein: each light receiver outputs a signal having a level related toamount of light received by said light receiver; and step (e) includesfine positioning the substrate, the shadow mask, or both until acombination of the levels of the signals output by the plurality oflight receivers equals a predetermined value or falls within apredetermined range of values.
 18. The method of claim 17, wherein thepredetermined value is zero.
 19. The method of claim 13, wherein: thesubstrate and the shadow mask each have a rectangular or square shapewith one grate adjacent each corner of the rectangle or square; and alongitudinal axis of each bar extends radially ±15 degrees from acentral axis of the corresponding substrate or shadow mask.